She may have been traumatized by something, or contracted an illness.
See the vet! I have heard this low growl a couple of times and it
isn't pretty.
Now, luring her out..Get down to her level and tempt her with catnip.
If that doesnt work (you have to get her out) use a broomhandle and
force her out. She'll thank you if the vet fixes her problem.

My vote would be to leave her alone for now.  She might have heard
something that was inaudible to you that really unnerved her.  Now she
needs a bit of time to chill out, and she's found the spot where she
wants to do it.

I wouldn't try to coax or bribe her, but rather ignore her, at least
until it is time to take her to the vet.  That low growl is meant as a
warning to be taken seriously.

I'll bet she will not turn out to be physically ill, but it never
hurts to find out.

Charlie

Get her out from under the bed.  Personally, I find the firing up a vacuum
cleaner will flush most cats out of hiding.  Of course, a really stubborn
cat may need the blower end of an old Electrolux aimed in its direction.
Works every time!

All the following comments are from personal experience, so take with
a grain of salt. Always best to call your vet and give em the
rundown,.

1. Is your house open in any way? Screen doors, screen windows,
recently opened due to the spring weather? It could be that she
smelled another cat and freaked out. How old is she? Could this be the
first time she encountered another strange cat?

2. Did you hear a yelp, like when you moved your chair or the other
cat was scrambling? Could be that she got injured and is panicked.
Cats are really good at avoiding possible injury - it's from their
wild roots - if you're a predator, injury can kill you because it
keeps you from hunting, or tips off others that you're unable to
defend yourself.This also seems to be why cats hide pain so well.

3. Any loud noises when she freaked? If she's young without the
experience of loud noises (in Minnesota, air-raid sirens traumatized
our young cat for weeks) then she might be reacting to something
unfamiliar.

Removing her:

Our good old cat "Terror" (survivor of a week in an airplane cargo
hold) would escape the house and hide under cars across the street. I
would hold out my hand and he would attack and attach himself like a
glove (I was young and felt indestructable, as all teens do). In that
pose we would stroll back to the house and bring him home. He was a
victim of lead poisoning (we found out later), so was irrational. If
your kitty is there for more than 24 hours then put on some tough
gardening gloves and go in after her.

I suspect that she'll creep out before then, though.

Comforting words, familiar toys, favorite treats may do a lot to
convince her there's no threat. If it was you who rolled over her tail
on a kitchen chair, regaining her trust might take slightly longer,
though. Lay out a prize/treat and watch from a safe distance. Remember
the capabilities of these animals to assess their surroundings.

Nonetheless, if she's otherwise healthy, she'll need to eat or use the
litterbox eventually. I couldn't scan the time of the OP, but I bet
this will correct itself. If not, call the vet, and most will visit
the house if there's an extreme situation.

Offered fwiw

BLink

snip

Thank you all for your help.  Here is a follow up.

Missy cat stayed under the bed all night.  She did come out on her own
this morning while I was preparing the cats breakfast.  She sniffed at
the food but didn't eat.  

She still isn't herself.  I don't think she drank and I'm sure she
didn't use the litter box.  She'll accept no treats either, and that's
a sure sign somethings wrong.  She did lie down on her pillow near
where I work on the computer and let me pet her, though she didn't
respond as she usually does.  At least she isn't in a scared, angry
and hiding mood any more.  

I took her to the vet and they didn't find anything obviously wrong.
They are keeping her for the day.  They want to see if she uses the
litter box, and will do some blood tests also.

Swyck

The vet didn't find anything wrong with Missy cat.  She spent the day
in observation.  Blood tests came up normal.  The doc said that she
may have been in some trauma, e.g. other cat at the window, scary
noise, or something, and he thinks she'll be fine.

She appears OK now, eating like normal, and doing her usual
activities.  Not sure what happened but I'm glad she's OK.  The vet
did say keep an eye on her.

It's Mr Spock that is having problems now.  He doesn't trust Missy cat
at all -- Its like she's a strange cat.  He always gets extremely
upset at other cats in the yard, and maybe she smells different from
being at the vet.  He hisses at her, won't come near her, and is
always watching her.

That could be an improvement over him beating her up every once in a
while.  He's the most vocal cat I've ever seen, and he's good for some
interesting vocalizations when he's mad.  I guess eventually, he'll
realize who she is, and that he can kick her butt, and probably will.

These cats are something else.

Swyck